Pros: More features than some full-sized amps, friendly and professional staff, outstanding warranty, good sound, easy to upgrade, great battery life

Cons: Perhaps a touch pricey, more 'transportable' than it is 'portable,' minor design/aesthetic flaws

My first review! I'm a bit of a newb, so please take what I say with a grain of salt!

The Great:

-This little amp is positively packed with features--and you'll use them all. Even the ones that you think you won't, like the 'flashlight' mode, or the 75 Ω button. (Makes for a great way to quickly lower the volume if someone is talking to you.)

-It's got some truly unique features--a lot of which are packed into the amp's power button. ('Sleep' mode, flashlight, LED dimmer, battery tester...) On top of that is the adjustable cross feed and the media control shuttle, which you can use to pause, play, and skip forward and back when you've got the amp hooked up to your computer. (Getting this to work the way you want it to may require some setup.)

-I think that the implementation of the bass and the treble boost is worth mentioning--even if you don't much care for EQing your headphones, it's hard to resist toying with these features a little bit. They add just enough oomph to make a noticeable and welcome difference in most cases. I've yet to turn them on with any music and headphones combination and be displeased with the result.

-The guys (or guy, or guys and girls, whatever) at Practical Devices are the very definition of great customer service. The unit I got came with a hex wrench that was too small to fully tighten the volume knob after taking it off to do some op-amp rolling. One email to Practical Devices and I got two wrenches of the correct size lickety split. (The amps now ship with the correct size wrench.)

-Three year warranty. Awesome.

-This amp is ridiculously easy to upgrade. Even an idiot like me can do it. Sure, I was sweating bullets the whole time, scared to death that I was going to irreparably break something, but it wound up being a piece of cake. The whole amp comes apart and goes back together again very easily, though sometimes getting the face plates re-positioned just right will cause a moment's aggravation.

-I love the battery life on this thing. Granted, its life does depend heavily on how the amp is configured, but stock (and even after some op-amp rolling) it still outlasts the battery on my iPod.

-The XM6 sounds good stock, and sounds even better when you install an op-amp that you like. Plenty of upgrade options. I, for one, am looking forward to trying other chips in it in the future.

The Good:

-The USB DAC is a nice little touch--I find myself using it quite a lot, especially at work. I stayed with the 8740 DAC in mine, though there are more choices when you purchase the amp.

-Other than a few niggly points (look below) I like the cross feed and use it fairly frequently--I find it makes long music-listening sessions considerably less fatiguing than they would be otherwise.

-While it's useless now, the AUX expansion should allow for some (hopefully) pretty cool add-on stuff in the future.

The Nitpicks:

-The adjustable cross feed is nice... but unless you carry a screwdriver with you everywhere you go, it's not the sort of thing you'll be using very often.

-Maybe it's just me, but I've found that it can be difficult to find a happy medium with the cross feed. It can sound and work great with most music, but with other music it can cause the center of some productions to get pretty muddy (Kayo Dot's Choirs of the Eye is a prime offender, for example), even when it's set fairly close to stereo. Now, I could take a screwdriver to the cross feed control and try to fine-tune it further, but when I'm on the go or actually trying to get some work done, it's just easier to turn the cross feed off. This is a problem that I don't have with my Headroom Micro Amp, and that's only got two flavors of cross feed: on or off.

-This is more of a transportable amp than it is a portable amp, really. It's not something that I'd try to use from my bag or a pocket--not only is it just a little too big for that, but it's really easy to push its buttons or turn its volume knob without trying. (Hasn't stopped me from using it this way before, though.) I think the best thing to do with it is to put on the adhesive feet that it came with and treat it as a portable desktop amp, if that makes sense.

-The XM6 appears to be solidly built, no denying that, but there are a few little aesthetic niggles I have with it. The groves on its side (on mine, at least) didn't get quite as good a coat of paint as did the rest of the amp. The buttons along the front of the device don't all line up totally right, and feel fairly loose and wiggly--though I doubt that they are going anywhere. Not sure that I care for the sticker that comes attached to it that displays your XM6's configuration--though you could probably remove it fairly easily if you wanted. (Can you tell that I'm really stretching to find faults at this point?)

-The XM6 is a bit on the pricey side when compared with some of its competition. I think the price is justified if you really plan on using all, or at least most of, the XM6's features. The fact that it's so easy to drastically change the amp's sound is really great, as well, and isn't something I see a lot of. Still, a lot of people can probably get what they need out of another amp and for quite a bit less. Just research before you buy.

In Sum:

I love this amp--it goes with me to work every day, and I use it at home if I'm away from my 'main rig' for whatever reason. If you've done your research, checked out the competition, have cash to spare, and love lots of features, go for it. The amp comes with a 30-day money back guarantee, so if it doesn't suit you at all, you're not out anything.

XM6: No other portable amp offers all this:
The XM6 is the most feature-packed portable headphone amplifier on the market today. Strong, clean, unsurpassed sound quality is made possible by the XM6's two-chip ultra-low-distortion FET amplification stage, mated to a separate two-chip output buffering stage, all in a monoblock configuration. All chips use gold-plated sockets, which allow for easy upgrading/rolling to configure the sound to exactly your taste. This attention to detail adds up to audio performance with an SNR well in excess of 100dB, and near-zero distortion (We are one of the few manufacturers who actually publish quantitative audio test results of our amplifiers. Test results here).
The XM6's USB-DAC stage is equipped with Texas Instruments' SRC4192 digital upconverter, feeding the legendary Wolfson Micro WM874x-series DAC (your choice of WM8740, WM8741, or WM8742), operating with 24-bit resolution at a 192kHz sampling rate. The result is absolutely unrivalled USB audio quality.
The XM6's features leave other amps far behind: there are controls for bass, treble, and gain boosts, and a control to enable/disable 75Ω output impedance (for those of you with lower-impedance headphones and IEMs). The XM6 also has Practical Devices' patent-is-pending Active Variable CrossFeed, which is simply not available on any other brand of amplifier. There is also a rear panel LineOut aux connector, and a first for any portable USB DAC: a multimedia controller joystick. And finally, the XM6 has intelligent microprocessor control, a 1-hour no-fuss Lithium-Ion FastCharge® battery system, auto power-off, and a low-battery indicator.
Lastly, the XM6 simply looks and feels spectacular. Available in your choice of color, the XM6 has robust, all-aluminum construction. It just looks, sounds, and feels spectacular. And all backed by an industry-leading 3-year warranty and no-hassles 30-day money-back satisfaction guarantee.